The present invention relates to a VLF test generator for generating a high voltage with a low frequency to test the insulation of capacitive loads, in particular power cables.
Testing with high voltages of very low frequency (VLF) in the range of a tenth of a hertz has become established for energy-related testing of the insulation of highly capacitive loads such as underground cable systems. In contrast to mains frequency or higher frequency test voltages, VLF test voltages only generate low reactive power in the capacitive load, and therefore the test generator can be of appropriately smaller design; and compared to d.c. voltage tests used previously VLF test voltages prevent the build-up of damaging space and residual charges in the cable system that can lead to disruptive breakdowns later during operation.
However, the generation of suitable VLF test voltages in the high voltage range, i.e. with up to some hundreds of kilovolts, is not at all simple, since high-voltage transformers are not practicable for such low frequencies. Therefore, a wide variety of switching systems have already been proposed for VLF test generators, which all, however, have either a high connection expenditure or components that are costly or fault-prone.
Thus, a VLF test generator of the aforementioned type is known from patent DE 103 33 241 B that uses a regulating transformer with motor-driven adjustment in order to modulate a mains frequency high voltage by periodic adjustment of the transformer in amplitude. The amplitude-modulated high voltage is stepped up and the modulation frequency is then recovered as VLF high voltage by means of a demodulator. The use of a regulating transformer operated by electric motor makes this solution unmanageable, fault-prone and only usable for low test power values.
Other known designs use complex high-voltage semiconductor switching systems in order to firstly rectify a mains frequency high voltage and then regulate it periodically by means of controlled semiconductor switches, in the manner of a high-voltage inverter as it were, in order to generate a VLF high voltage. While these designs manage without mechanical components, they still require extensive power electronics that also have a high cooling requirement.
The aim of the invention is to overcome the disadvantages of the known prior art and provide a VLF test generator for the generation of low-frequency high voltages, which can be achieved simply and inexpensively, is low in weight for a transportable use on site, is sturdy and not fault-prone and has only a low cooling requirement.